1. Field of the Invention
The present invention relates generally to a variable oscillator, and more particularly to a novel temperature compensated variable oscillator using lithium tantalate as its resonator, or vibrator.
2. Description of the Prior Art
In the art, the crystal resonator, or vibrator, which is used as an oscillating element is operated in a frequency range within which the reactance of the crystal vibrator is inductive. However, its reactance variation or inductance variation sharply varies in this frequency range, as is well known. Accordingly, an oscillator circuit employing the crystal vibrator can produce a stable oscillation frequency for temperature changes, but can not be used as a variable frequency oscillator circuit with a broad band of frequency variations such as, for example, an FM modulator circuit, because only a small range of variation in oscillation frequency can be obtained from the inductance variation of the vibrator.
A single crystal vibrator of lithium tantalate (LiTaO.sub.3), which is a piezo-electric element, has an inductance component which is greater than that of the crystal vibrator, on the order of several tens of times. Hence the inductance variation of the lithium tantalate vibrator within the aforenoted frequency range is gradual rather than sharp, so that the range of variation in oscillation frequency is widened. As a result, if lithium tantalate is used as an oscillating element, the above mentioned FM modulator circuit can be realized.
A typical example of such a variable oscillator circuit has been proposed. This proposed circuit is shown in FIG. 1 of the accompanying drawings at 10. The illustrated variable oscillator circuit 10 is connected as a pierce type oscillator. In FIG. 1, reference numeral 1 designates a vibrator made of lithium tantalate LiTaO.sub.3 as the oscillating element, Q is a transistor for oscillation, 2 and 3 are capacitors for oscillation, and 11 is an input terminal which is supplied with a modulation signal such as, for example, an audio signal. A variable capacitance 12, such as a variable capacity diode, is connected to the vibrator 1 of LiTaO.sub.3, the capacitance being varied by the modulation signal to vary the oscillation frequency f.sub.o of the circuit. Reference numeral 13 indicates an output terminal.
When the variable oscillator circuit 10 is included in an FM modulator circuit, the FM modulator, for both general use and for the specific use in a wireless microphone desirably should satisfy the following conditions:
I. the oscillation frequency (carrier frequency) f.sub.o can be selected from a wide range. PA1 Ii. the frequency deviation at the desired (used) oscillation frequency f.sub.o can be relatively large. PA1 Iii. the temperature dependent frequency characteristics are good.
The condition I is especially necessary for the application wherein the variable oscillator circuit is employed in the wireless microphone. That is, when the variable oscillator circuit 10 is used as the FM modulator circuit in the manner described above, its oscillation frequency f.sub.o is generally selected within a carrier frequency band between 76 MH.sub.Z and 90 MH.sub.Z. If the oscillation frequency f.sub.o is selected at, for example, 78 MH.sub.Z , this oscillation frequency f.sub.o may be obtained directly from the oscillator circuit 10 itself or by choosing the oscillation frequency of the oscillator circuit 10 to be lower than the required frequency and then multiplying it by an appropriate factor. When the carrier frequency is selected to be 78 MH.sub.Z as above, an oscillation frequency f.sub.s of the oscillator circuit 10 may be set at 26 MH.sub.Z and then this frequency should be multiplied by the factor of 3 to obtain the necessary carrier frequency f.sub.o of 78 MH.sub.Z.
Accordingly, if such a wireless microphone is used in an area where the electric field strength or intensity of surrounding FM broadcasting waves is relatively strong; and if the carrier frequency derived from the oscillator circuit 10 is selected to be close to an FM broadcasting frequency, then although the oscillator circuit output signal is received by an FM radio receiver with an AFC (automatic frequency control), only the FM broadcasting wave is processed due to the fact that the FM broadcasting wave has stronger electric field strength. Consequently, there is a strong possibility that the output signal from the oscillator circuit 10 can not be satisfactorily received by the FM radio receiver. Further, if the oscillation frequency f.sub.o is even closer to the carrier of the FM broadcasting wave, it is quite possible that an interference occurs therebetween.
Therefore, it is desired that the oscillation frequency f.sub.o or f.sub.s be capable of being varied to some extent. A variable frequency range .DELTA.f of about 2 .about. 3 MH.sub.Z will be sufficient for practical use.
The frequency range, within which the lithium tantalate (LiTaO.sub.3) vibrator 1 becomes inductive, is generally limited to about 1.6 MH.sub.Z at the oscillation frequency f.sub.s of 26 MH.sub.Z, so that the full extent of the above variable frequency range .DELTA.f can not be presented by the circuit construction illustrated only in FIG. 1. In addition, for the case where the oscillation frequency f.sub.s is taken as the carrier frequency and is modulated with the audio signal (modulating signal) derived from the wireless microphone, even if the capacitors 2 and 3 shown in FIG. 1 are changed so as to change the oscillation frequency, the desired variable frequency range .DELTA.f noted above can not be obtained. For the desirably large variable frequency range, the variable capacity range of the capacitors must be in the range of several pF to several hundred pF, which can not be realized.
However, if, as shown in FIG. 1, an inductance element such as a coil 4 in the illustrated example is connected in series to the vibrator 1 of LiTaO.sub.3 and its inductance is varied, the reactance characteristics of the oscillator circuit can be varied sufficiently. Thus, the above conditions I and II can be satisfied.
FIG. 2 of the accompanying drawings is a graph showing the admittance characteristic of the lithium tantalate vibrator 1 in which the abscissa represents an oscillation frequency f and the ordinate the admittance Y. In the graph of FIG. 2, f.sub.a indicates a parallel resonance frequency, as is well known, and f.sub.r a series resonance frequency. Accordingly, an oscillation frequency within a frequency range determined by a variation curve l.sub.1, where the reactance becomes inductive, is generally used, which is a frequency range of about 1.6 MH.sub.Z as described above. If, in the above graph of the characteristic curves, the inductance of the coil 4 is varied, the variation curve is changed from a curve l.sub.2 to a curve l.sub.4 whereby the series resonance frequency f.sub.r is lowered as the inductance value is increased and hence the frequency range, within which the reactance becomes inductive, is increased. For this reason, by adjusting the inductance value, an arbitrary frequency in the desirable variable frequency range of .DELTA.f can be used, and the inclination or gradient of the variation curve becomes gradual. As a result, the variable frequency range for selecting the desired frequency can be made great. If the oscillation equivalent capacity C.sub.o is changed by, for example, .+-..DELTA.C.sub.o, the oscillation frequency is changed to f.sub.s .+-..DELTA.f.sub.s. Thus, in this example, .+-..DELTA.f.sub.s is the variable frequency range and hence is greater than that of the curve l.sub.1. In this case, the oscillation equivalent capacity C.sub.o can be expressed as follows if the capacities of the first and second capacitors 2 and 3 and the capacity of the variable capacity diode 12 are taken C.sub.1, C.sub.2 and C.sub.D, respectively. ##EQU1##
If the inductance is made variable, the variable frequency range can be made sufficiently great so that the frequency range of 2.about.3 MH.sub.Z can be easily obtained by suitably selecting the inductance value as described previously. Therefore, the above conditions I and II can be easily satisfied by the provision of the coil 4 only.
If the variable oscillator circuit 10 is constructed by using the coil 4, then due to the fact that the vibrator 1 of lithium tantalate itself has a temperature characteristics which varies in a second order relationship so as to be graphically represented by a second degree curve opening upward and also that the coil 4 itself has temparature characteristics, the variation of the oscillation frequency in response to temperature changes becomes substantial with the result that a highly stable variable oscillator circuit can not be obtained. That is, the above condition III can not be satisfied.
In such a case, the coil 4 has such a temperature characteristic that its inductance value linearly changes greatly in response to the temperature change which is caused by the fact that the magnetic permeability of the core (ferrite core) of the coil 4 changes greatly in response to the temperature change. Therefore, if the oscillation frequency f.sub.s is selected at 26 MH.sub.Z and the oscillation frequency f.sub.s is deviated between +400 KH.sub.Z (= f.sub.s + .DELTA.f = f'.sub.s) and -600 KH.sub.Z (= f.sub.s - .DELTA.f'.sub.s = f".sub.s), the temperature characteristics of the oscillation frequency can be shown in the graph of FIG. 3 of the accompanying drawings. In the graph of FIG. 3, a curve 15a shows the temperature characteristic at the frequency of f.sub.s, a curve 15b the temperature characteristic at the frequency of f'.sub.s and a curve 15c the temperature characteristic at the frequency of f".sub.s, respectively. As may be apparent from the graph of FIG. 3, as the temperature is varied from +20.degree.C to +40.degree.C, the variation of about 300 PPM (which corresponds to .+-. 7.8 KH.sub.Z in frequency variation) occurs. As a result, it may be apparent that if the oscillator circuit is used in the above construction, there is the attendant problem of stability of the frequency.